1. Field of the Invention
This invention pertains to a lighttight cassette for a roll of web material, with a housing that has a dispensing slot formed by two housing sections. The slot runs essentially parallel to the axis of the roll and has an adjustable width.
2. Description of Related Art
This type of cassette is loaded, as a rule, with light-sensitive material wound on a roll and must fulfill several, partially contradictory requirements. The cassette must also be locked lighttight in the area of the dispensing slot. On the other hand, the web material, for example, a light-sensitive film, should not be scratched on being withdrawn from the cassette; that is, in the area of the dispensing slot, the material should be passed through the slot without being pinched or abraded by the housing. Furthermore, the forces required to unroll the material uniformly should not be too high, because this adversely affects smooth transport of the material. On the other hand, the beginning of the web should not be retracted unintentionally into the cassette during transport of the cassette.
For cost reasons, such a cassette should also be suitable for various thicknesses of the web, such as film or paper; that is, the dispensing slot width must be adapted to each web thickness.
In a known cassette (German 29 50 624 A1), the housing sections are under preloaded spring tension and are brought together with spring action. Therefore, the leading end of the film is normally clamped in the dispensing slot. A tug on the film applies on the one housing section a force that is directed so as to widen the slot. The slot then becomes wide enough so that the film contacts only the lower housing section. This prevents the coated outer side of the moving film from coming into contact with the other housing section and being scratched. In this design, the risk of the coated side of the film being scratched is indeed relatively low. Conversely, the risk of the other side of the material being scratched is that much higher. In order to be able to actually withdraw the film from the cassette, considerable tensile stress is required. On the one hand, this must accomplish transporting the film out of the cassette, and on the other hand, this must overcome the spring closure force of the lower cassette section in order to hold the dispensing slot open.
The object of the present innovation is to provide a lighttight cassette that enables uniform unwinding of web material of various thicknesses without scratching, even in exposure devices, for example, scanners, that apply very low tensile stress.